1. Field of the Invention
The present invention relates to an optical storage medium and an information recording/reproducing apparatus for recording or reproducing information with respect to the optical storage medium. More particularly, the present invention relates to an optical storage medium in which recording signals of images, sounds and the like are encrypted for protecting works from improper duplication, and relates to an information recording/reproducing apparatus for recording or reproducing information with respect to such an optical storage medium.
2. Related Background Art
Optical discs exemplified by DVDs (Digital Versatile/Video Discs) have become widespread as a medium for recording massive amounts of digital data such as AV (Audio Visual) data, a medium used for computers and the like. For example, DVDs in which high quality moving images are recorded for two hours or longer are now on the market. In order to protect such digital works against improper copying to another recording medium, a method called contents encryption is adopted (See ITE Technical Report Vol. 21, No. 31, pp. 15 to 19, for example).
According to this method, compressed digital works such as a movie are encrypted by means of secret keys with a three-level hierarchy (title key, disc key and master key) and are recorded in a user region to which a user is allowed to access. Then, only the authorized maker having a license is notified of the master key that is the most important key among the secret keys. The disc key and the title key that are necessary for each DVD and each title are encrypted based on the master key and are stored in a control information region to which a user is not allowed access. With this configuration, a restriction is imposed on the user so as not to access the secret keys required for decoding, and therefore improper copy by means of file copying and the like can be prohibited. However, such technology cannot cope with an improper action in which contents of all regions including the control information region for recording the secret keys are improperly copied in their entirety to another optical disc.
To address this, a method for judging whether an optical disc is an authorized one or an improperly copied one is suggested as shown in FIG. 53, where recording marks in the optical disc are displaced in units of a frame (A of FIG. 53) in the radial direction so as to record a singular pit row (B of FIG. 53) (See JP 3061098 B, for example).
The presence of such a singular pit row can be detected by a burst TE signal (D of FIG. 53) that is generated from a tracking error signal (C of FIG. 53). Therefore, by counting the number of frames in which a signal level of a standard signal that is generated internally and that of the burst TE signal agree with each other, the optical disc can be distinguished between the authorized medium and the copied medium based on this counter value.
When this optical disc in which recording marks are displaced in the radial direction is copied by an ordinary method, the singular pits cannot be generated in the copied optical disc. Therefore, it becomes possible to find an improperly copied optical disc.
However, when pit rows are recorded so as to wobble, the tolerance for offtrack will be narrowed correspondingly, thus reducing the reliability of the optical storage medium and an information reproducing apparatus. In order to secure the reliability, a pitch of tracks for recording the pit rows has to be widened by the size of the wobble, thus decreasing the storage capacity.
In addition, according to the above conventional technology, judgment on the authorized disc is conducted depending on the presence of singular pits only, and therefore if a location of the singular pits is determined, improper copying can be conducted by generating any singular pit row at that location. As a result, unauthorized disc makers who do not have a license can manufacture a large amount of copied digital works, which might hinder the appropriate distribution of digital contents.